1. Field of the Invention
The present invention relates to a light emitting device, and more particularly to two AlGaInP light emitting diodes in series connection by semiconductor manufacture process.
2. Description of the Prior Art
The conventional AlGaInP LED has a double hetero-structure (DH), as shown in FIG. 6. The LED stacked sequentially, from a bottom thereof, has an n-type ohmic contact electrode 2, a GaAs substrate 3, an n-type (AlxGa1−x)0.5In0.5P lower cladding layer 4 with an Al composition between about 70%–100%, an (AlGa1−x)0.5In0.5P active layer 5 with an Al composition of 0%–45%, a p-type (AlxGa1−x)0.5In0.5P upper cladding layer 6 with an Al composition 70%−100%, a p-type high energy band gap current spreading layer 7 such as layers of GaP, GaAsP, AlGaAs or GaInP, and a p-type ohmic contact layer 8 as well as a bonding pad 9.
With the composition alternation of the active layer 5, the wavelengths of the light emitted are varied from 650 nm: red to 555 nm: green. A drawback is generally found in the conventional LED, that is: while the light emitted from the active layer 5 towards the substrate 3 will be totally absorbed by GaAs substrate 3. It is because the GaAs substrate has an energy gap smaller than that of the active layer 5. Therefore, the light generated is absorbed resulted in lower light generated efficiency for this kind of conventional AlGaInP LED.
To overcome the substrate 3 light absorption problem, several conventional LED fabrication technologies have been disclosed. However, those conventional technologies still accompany with several disadvantages and limitations. For example, Sugawara et al. disclosed a method published in Appl. Phys. Lett. Vol. 61, 1775–1777 (1992), Sugawara et al. inserted a distributed Bragg reflector (DBR) layer in between GaAs substrate and lower cladding layer so as to reflect those light emitted toward the GaAs substrate. However, the reflectivity of DBR layer is usefully only for those light which almost vertically towards the GaAs substrate. With the decrease of injection angle, the reflectivity is drastically decreased. Consequently, the improvement of external quantum efficiency is limited.
Kish et al. disclosed a wafer-bonded transparent-substrate (TS) (AlxGa1−x)0.5In0.5P/GaP light emitting diode, entitled “Very high efficiency semiconductor wafer-bonded transparent-substrate (AlxGa1−x)0.5In0.5P/GaP” on Appl. Phys. Lett. Vol. 64, No. 21, 2839 (1994). The TS AlGaInP LED was fabricated by growing a very thick (about 50 μm) p-type GaP window layer by hydride vapor phase epitaxy (HVPE) formed on epi-layers light emitting structure. Subsequently, the temporary n-type GaAs substrate is selectively removed using conventional chemical etching techniques. After removing the GaAs substrate, the LED epilayer structure is then bonded to an 8–10 mil thick n-type GaP substrate.
For the light illuminated concerned, the TS AlGaInP LED exhibits a two fold improvement in light output compared to absorbing substrate (AS) AlGaInP LEDs. However, the fabrication process of TS AlGaInP LED is very complicated. Since the bonding process is to make two III–V semiconductor wafers directed bond together by heating and pressing for a period of time. Even worse, a non-ohmic contact interface between them is generally found to have high resistance. To manufacture these TS AlGaInP LEDs in high yield and low cost is difficult as a result.
Another conventional technique was proposed by Horng et al., on Appl. Phys. Lett. Vol. 75, No. 20, 3054 (1999) entitled “AlGaInP light-emitting diodes with mirror substrates fabricated by wafer bonding.” Horng et al., reported a mirror-substrate (MS) of AlGaInP/metal/SiO2/Si LED fabricated by wafer-fused technology. In LED, AuBe/Au stack layer function as a bonding layer for silicon substrate and epi-layer LED. However, the intensity of the AlGaInP LED is only about 90 mcd under 20 mA injecting current. The light intensity is at least lower than that of TS AlGaInP LED by 40%. It could not be satisfied.